The linear motor has a magnetic field part having a plurality of permanent magnets and an armature arranged in an opposite manner on the magnetic field part with a gap created therebetween. In the magnetic field part, the permanent magnets are arranged in a straight line in such a manner the N poles and S poles are formed alternately. The armature has a core having a plurality of teeth opposing to the permanent magnets of the magnetic field part and a plurality of coils wound on the respective teeth. When alternate current is made to pass through the phase coils wound on the respective teeth, there occurs a moving magnetic field. This moving magnetic field and the magnetic field of the permanent magnets interact with each other, which generates a thrust so that the armature linearly moves relative to the magnetic field part.
In a linear motor that moves linearly, the length of the armature is limited in the moving direction, while the armature of a rotary motor is formed endless. Therefore, when the armature moves relative to the magnetic field part, there likely occurs cogging. Cogging is a phenomenon of magnetic forces between the core of the armature and permanent magnets pulses depending on the electrical angle.
Generally, the core is made of a magnetic material. When current does not flow into the coil, the magnetic attraction force is generated between the teeth of the core and the permanent magnets. When the armature moves relative to the magnetic field part, the teeth of the core are attracted by permanent magnet in front or retracted by permanent magnet in the rear. This is considered to cause such a cogging that the magnetic attraction force applied to the armature varies periodically in every magnetic pole pitch of the permanent magnets. When the current is passed through the coils, the cogging remains and acts as external disturbance.
In a conventional art, there are known auxiliary magnetic poles provided at both ends of the core of the armature in the moving direction in order to cancel the cogging (see the patent literature 1). On each of the auxiliary magnetic poles at both ends, no coil is wound. The distance between the auxiliary magnetic poles at both ends is set to such a distance that magnetic attraction forces generated at the respective ends cancel each other.